The role of chromatin in transcriptional regulation.

Transcriptional activation is mediated by the facilitated binding of the basal transcription complex to the transcription start site of a promoter. The activation procedure involves protein-protein interactions between specific transcription factors and members of the basal transcription complex. However, since eukaryotic DNA is packaged with histones into nucleosomes the accessibility of the transcription factors is limited. In order to activate transcription, some of the specific transcription factors must have the capacity to bind to their binding sites when organized into nucleosomes. As a next step, the chromatin structure of the promoter needs to be decondensed in order to facilitate the binding of the basal transcription machinery. Recent data have addressed these issues and both binding of transcription factors to their chromatin binding site as well as transcription factor-induced chromatin remodelling have been demonstrated. In addition, factors that are candidates to mediate the chromatin remodelling have recently been identified and characterized. The ability of a transcription factor to recognize its cognate element in a nucleosome is an inheret property that differs among different transcription factors. The implications of the rotational and translational positioning of the DNA within a nucleosome on the accessibility of a transcription factor is described in this review. In addition, nucleosome rearrangement and juxtaposing in the context of transcriptional activation is also discussed.